Conventionally, a solvent deasphalting (SDA) unit is employed by an oil refinery for the purpose of extracting valuable components from vacuum residual, or residual oil, which is a heavy hydrocarbon produced as a by-product of refining crude oil. The extracted components are fed back to the refinery wherein they are converted into valuable lighter fractions such as gasoline, etc.
In a typical SDA unit, a light hydrocarbon solvent such as propane, iso- or normal butane, iso- or normal pentane, or mixtures thereof, is added to the heavy hydrocarbon feed from a refinery and applied to what is termed an asphaltene separator. Under elevated temperature and pressures, the mixture in the separator separates into a plurality of liquid streams, typically, a substantially asphaltene-free stream of deasphalted oil (DAO) and solvent, and a mixture of asphaltene and solvent within which some DAO may be dissolved. Sometimes, one or more substantially asphaltene-free streams of resin and solvent are also produced by treating the DAO with another lighter solvent, or by heating, or depresurizing the stream of DAO and solvent before supplying the stream to another separator.
The solvent recovery section of an SDA unit extracts essentially all of the solvent from these streams producing a solvent-free DAO product stream, and solvent-free asphaltene product stream. The DAO stream usually is returned to the refinery for conversion to gasoline, jet fuel, etc., and the asphaltene stream usually is combined with diluent, such as diesel fuel, for conversion to residual fuel oil.
In some installations, the solvent recovery operation includes a supercritical solvent recovery section that removes a large percentage of solvent from the stream of DAO and solvent, followed by an evaporative solvent recovery section that removes the remaining solvent from the DAO, and all of the solvent from the stream of asphaltene and solvent. In other installations, only an evaporative solvent recovery section is used. In both cases, the output of the evaporative solvent recovery section is DAO product and asphaltene product having acceptable levels of solvent (e.g., less than about 0.05% by weight).
In an evaporative solvent recovery section, each of the liquid product streams of DAO and solvent, or asphaltene and solvent, is first flashed to produce a vaporized solvent stream, and a reduced solvent liquid product stream. Each reduced solvent liquid product stream so produced is then subjected to serial flashing and/or stripping until the final product stream is free of solvent to the desired degree. The vaporized solvent produced in this manner is condensed and re-used by application to the hydrocarbon feed. An example of an evaporative solvent recovery section is disclosed in copending applications Ser. No. 08/618,570 filed Mar. 20, 1996, the disclosure of which is hereby incorporated by reference.
Vacuum residual produced by a refinery is a heavy viscous material that is usually solid at room temperature complicating its storage and transport. By mixing vacuum residual with atmospheric distillate (e.g., kerosene, diesel fuel, etc.) produced by a refinery, the viscosity of the mixture is reduced thereby facilitating its transportation. However, the presence of atmospheric distillate, from any source, in the heavy hydrocarbon feed applied to an SDA unit results in the trapping of the distillate in the solvent loop.
The problem is that some of the atmospheric distillate passes out of the bottom of the asphaltene separator of the SDA unit with the asphaltenes/solvent stream, and the remainder of the atmospheric distillate passes out the top of the separator with the DAO/solvent stream. Recovery of the solvent in the evaporation process effected by the unit also results in recovery of the atmospheric distillate which remains trapped in the solvent. Without an outlet for the atmospheric distillate, the concentration thereof in the solvent increases over time inhibiting phase separation in the asphaltene separator. At some point, the concentration of atmospheric distillate becomes so large that phase separation ceases. When this occurs, the unit has to be shut down and the trapped atmospheric distillate removed from the system.
Front-end flashing of the feed before it is applied to an SDA unit is not normally effective to remove sufficient atmospheric distillate from the feed to solve the problem. Subjecting the feed to fractional distillation would be a solution. However, the cost of either of these expedients is prohibitive.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for removing atmospheric distillate present in a heavy hydrocarbon feed containing asphaltenes during a solvent deasphalting operation.